1. Field
The embodiments discussed herein are relates to a semiconductor integrated circuit including a control circuit that supplies power to a circuit including a plurality of circuit blocks having different power systems.
2. Description of the Related Art
As metal-oxide semiconductor (MOS) transistors included in semiconductor integrated circuits are becoming more and more minute and detailed, the number of gates that can be mounted on a semiconductor integrated circuit is increasing. As the number of mounted gates included in the semiconductor integrated circuit increases, power consumed by the entire circuit increases. In order to reduce power consumption, a so-called power gating circuit has been adopted. This is a power control circuit that connects circuit blocks included in the semiconductor integrated circuit to a power supply when the circuit blocks are being used and disconnects the circuit blocks from the power supply when the circuit blocks are not being used.
Circuit blocks that use the same power supply are called power domains. A general power gating circuit includes power switches that connect/disconnect corresponding power domains including circuit blocks to/from the power supply of the entire semiconductor integrated circuit, a power management unit (PMU) that outputs control signals for controlling turning on/off of the power switches, and an isolator circuit that prevents a signal from one power domain disconnected from the power supply of the entire semiconductor integrated circuit to the other power domain connected to the power supply of the entire semiconductor circuit from entering a high impedance (Hi-Z) state, that is, a floating state (for example, see Japanese Unexamined Patent Application Publication No. 2006-344640).
However, the isolator circuit is not partially included in the following case. For example, when a circuit block A belonging to one power domain is operating, a circuit block B belonging to the other power domain operates for sure in association with the operation of the circuit block A. If the opposite relationship does not hold true, no power control is performed to prevent, when one power domain is connected to the power supply of the entire semiconductor integrated circuit, the other power domain from being connected to the power supply of the entire semiconductor integrated circuit. In that case, the power gating circuit includes no isolator circuit from the circuit block B to the circuit block A in order to reduce the number of gates.
In such a case, an incorrect power control sequence may be set in the PMU or a central processing unit (CPU) controlling the PMU because installing a control program into the PMU or CPU is separately done after completion of fabrication of the semiconductor integrated circuit. Thus, when power is being supplied to the circuit block A but no power is being supplied to the circuit block B, a signal from the circuit block B to the circuit block A becomes floating, which causes a failure of the circuit block A.